The present invention relates to a gated semiconductor device having a field plate termination.
For MOS gated semiconductor devices such as field effect transistors, the standard method of termination is to continue source metal beyond the periphery of an active area of the device so as to form a field plate extending around the periphery of the device In an N-channel MOSFET, when a positive bias is applied to the drain whilst the gate and source are connected to the ground potential the transistor is in the forward-biased off state. This creates a depletion region which is brought to tie surface more gradually at the edge of the device than at the centre due to the presence of the field plate formed from the continuation of the source metal. This reduces the peak electric field at the periphery of the device, thus increasing the breakdown voltage.
One requirement of a MOSFET device is to connect the gate to a gate bond pad. The gate is normal made of polysilicon which has a significant resistance that combines with the device capacitance to create a time constant RC. In order to give acceptable switching characteristics it is thus necessary to break the source metal by distributing metal gate electrodes throughout the structure and/or around the periphery of the device. Some MOSFETs use a striped rather than a matrix polysilicon design, in which case the gate electrode may also be needed to connect the polysilicon gate stripes together. The larger the gate electrode area, the faster the switching, but also the higher the on resistance, and so there needs to be a trade-off between speed and resistance.
It is known to combine the functions of a field plate and a gate electrode by forming a conductive layer separated from the substrate by an insulating layer and extending around the active surface area between the field plate and gates which are positioned around the periphery of the active area. In this type of arrangement, the conductive layer and the peripheral gates may be formed from portions nf a single layer of polysilicon on which a field plate conductor is deposited.
These known devices in which the field plate and peripheral gates are interconnected can be damaged or destroyed however by electrostatic discharge or circuit transients. An electrostatic discharge (e.g. from a human being) can charge up the gate to a potential beyond its rupture voltage, thus destroying or irreparably damaging the insulating layer which separates the gate from the substrate.